1. Field of the Invention
This invention lies in the field of high-sensitivity gas analysis.
2. Description of the Prior Art
Persons suffering from asthma are able to monitor the intensity of their condition and to predict the likelihood of an asthmatic attack by monitoring the level of nitric oxide (NO) in their exhaled breath. Sensors that detect nitric oxide levels in exhaled breath are disclosed in U.S. Pat. No. 5,795,787, issued Aug. 18, 1998, U.S. Pat. No. 6,010,459, issued Jan. 4, 2000, United States Pre-Grant Publication No. US 2004-0017570 A1, published Jan. 29, 2004, United States Pre-Grant Publication No. US 2005-0083527 A1, published Apr. 21, 2005, and United States Pre-Grant Publication No. US 2005-0053549 A1, published Mar. 10, 2005. Each of these documents is incorporated herein by reference in its entirety. The sensors described in these documents are contained in devices with flow-through passageways for retention of nitric oxide-binding materials, allowing the user to exhale into the device and obtain a reading from the device that indicates the nitric oxide level in the exhaled breath. The sensors themselves use sophisticated sol-gel technology in conjunction with nitric oxide biding agents such as cytochrome C and other proteins that bind nitric oxide upon contact and undergo optically detectable changes, most notably optical absorption, when such binding occurs. The sensors that have been developed under this technology are sensitive enough to detect nitric oxide in the parts-per-billion range.
Such a high level of sensitivity makes the sensors vulnerable to interferents that are present in extremely low quantities, including gaseous interferents that are emitted from the materials of construction of the device itself. It has indeed been discovered that monitoring devices that contain these sensors in certain polymeric housings, notably housings made from acrylic resins, display a gradual loss in sensitivity over time. The loss in sensitivity limits the shelf lives of these devices and hence the suitability of these devices for long-term storage for purposes of long-distance shipping and for the building of inventories adequate to meet growing or fluctuating demands. This loss of sensitivity is due to the emission of low levels of carbon monoxide by the polymeric material from which the device housing is constructed. The reason for the carbon monoxide emission is unknown, and may be a de-gassing of the polymer to release dissolved carbon monoxide, a decomposition of additives included in the polymer, or a decomposition of the polymer itself. Each of the terms “emission” and “release” is used herein to generically cover all of these possible mechanisms. Regardless of the reason for the emission, the cumulative adsorption of the emitted carbon monoxide by the sensing material is believed to cause a reduction in the binding sites available for nitric oxide. While not asserting this explanation is incontestable, the inventors herein offer it as a possible explanation for the loss in sensitivity. Similar concerns arise with other nitric oxide sensing devices that are constructed with carbon monoxide-emitting polymers, including devices that are designed for atmospheric testing or for physiological testing other than exhaled breath, such as nasal emissions. In many of these devices, nitric oxide detection is needed at concentrations higher than those encountered in the exhaled breath of an individual suffering from asthma. Nevertheless, these devices can also be susceptible to a loss of sensitivity for the same reasons as the exhaled breath analyzers cited above.
The prior art recognizes the existence of transition metal oxides that can catalyze the oxidation of carbon monoxide to carbon dioxide, and it is also known that carbon dioxide does not bind to proteins with the same affinity as carbon monoxide. Disclosures of these metal oxides and their use as oxidation catalysts appear in U.S. Pat. No. 4,818,745, issued Apr. 4, 1989, U.S. Pat. No. 5,955,214, issued Sep. 21, 1999, U.S. Pat. No. 6,113,869, issued Sep. 5, 2000, U.S. Pat. No. 6,203,596 B1, issued Mar. 20, 2001, and U.S. Pat. No. 6,855,297 B2, issued Feb. 15, 2005. Each of these documents is likewise incorporated herein by reference in its entirety. The same metal oxides can catalyze the oxidation of nitric oxide to nitrogen dioxide (NO2), however. Accordingly, the removal of the interferent by oxidation can also be expected to cause removal of the analyte and therefore to interfere with the nitric oxide detection.